The effect of carbon coating of AIN powder on sintering behavior and thermal conductivity

High thermal conductive AlN ceramics doped with Y₂O₃ were produced by sintering the powders obtained after applying a carbon coating to the surface of AlN powder grains. During sintering at 1800°C for 1 hour, the carbon reacts with the surface of the AlN grains by carbothermal-reduction of Al₂O₃, and also with the Al₂Y₄O₉ intermediate phase to form AlN, Y₂O₃ and CO. By adding 0.56 mass% of carbon, almost all the Al₂Y₄O₉ is reacted and the thermal conductivity increases from 184 W/(m · K) to 224 W/(m · K). Further carbon addition decreases the thermal conductivity and also the final sintered density.     

Hot-pressed AlN-Cu metal matrix composites and their thermal properties

AlN-Cu metal matrix composites containing AlN volume fractions between 0.1 and 0.5 were fabricated firstly by liquid phase sintering of AlN using Y₂O₃ as a sintering aid and then by hot pressing the powder mixtures of sintered AlN and Cu at 1050°C with a pressure of 40 MPa under flowing nitrogen. With Y₂O₃ additions of 1.5 to 10 wt%, the densification of AlN could be achieved by liquid phase sintering at 1900°C for 3 h and subsequently slow cooling. The sintered AlN showed a maximum thermal conductivity of 166 W/m/K at a Y₂O₃ level of 6 wt%. Dense AlN-Cu composites with AlN contents up to 40 vol% were achieved by hot pressing. The thermal conductivity and the coefficient of the thermal expansion (CTE) of the composites decreased with increasing AlN volume fractions, giving typical values of 235 W/m/K and 12.6 × 10^–6/K at an AlN content of 40 vol%.     

The use of yttrium (III) isopropoxide to improve thermal conductivity of polycrystalline aluminum nitride (AlN) ceramics

Instead of Y₂O₃ powders, yittrium isopropoxide (YIP) was used as a sintering additive to sinter high thermal conductivity polycrystalline aluminum nitride (AlN). The reasons for using sintering additive in sol-gel form are due to the fact that the particle sizes are uniform in the nano scale and also they promote a better coating of AlN grains, being more effective during sintering process. The binder burn out was carried in two different atmospheres, N₂ (N₂ BBO) and air (air BBO). The thermal conductivity of dense polycrystalline aluminum nitride samples with the addition of Y₂O₃ (YIP formulation) ranging from 1.0 to 10.0 wt% with N₂ BBO and air BBO was measured by the laser-flash technique. The results of measured thermal conductivity exhibited higher values than those reported for samples of same yttria formulation (Y₂O₃ powder) and sintered conditions.     

CaO-Y2O3添加剂对AlN陶瓷显微结构及性能的影响

研究了掺杂ＣａＯ－Ｙ２Ｏ３热压烧结和常压烧结ＡｌＮ陶瓷的性能和显微结构．结果表明：热压烧结ＡｌＮ陶瓷的第二相为Ｙ３Ａｌ５Ｏ１２，常压烧结ＡｌＮ陶瓷的第二相为Ｙ３Ａｌ５Ｏ１２和Ｃａ３Ｙ２Ｏ６；热压烧结ＡｌＮ的第二相体积百分数和晶格氧含量均低于常压烧结；热压烧结ＡｌＮ陶瓷的微观结构良好，其热导率达到２００Ｗ／ｍ·Ｋ．     

Microstructure and properties of spark plasma sintered AlN ceramics

Spark plasma sintering (SPS) is a newly developed technique that enables poorly sinterable aluminum nitride (AlN) powder to be fully densified. It is addressed that pure AlN sintered by SPS has relatively low thermal conductivity. In this work, SPS of AlN ceramic was carried out with Y₂O₃, Sm₂O₃ and Li₂O as sintering aids. Effects of additives on AlN densification, microstructure and properties were investigated. Addition of sintering aids accelerated the densification, lowered AlN sintering temperature and was advantageous to improve properties of AlN ceramic. Thermal conductivity and strength were found to be greatly improved with the present of Sm₂O₃ as sintering additive, with a thermal conductivity value about 131 Wm⁻¹K⁻¹ and bending strength about 330 MPa for the 2 wt% Sm₂O₃-doped AlN sample SPS at 1,780 °C for 5 min. XRD measurement revealed that additives had no obvious effect on the AlN lattice parameters. Observation by SEM showed that AlN ceramics prepared by SPS method manifested quite homogeneous microstructure. However, AlN grain sizes and shapes, location of secondary phases varied with the additives. The thermal conductivity of AlN ceramics was mainly affected by the additives through their effects on the growth of AlN grain and the location of liquid phases.     

Microstructure and thermal conductivity of spark plasma sintering AlN ceramics

Abstract

Dense aluminium nitride ceramics were prepared by spark plasma sintering at a lower sintering temperature of 1700°C with Y₂O₃, Sm₂O₃ and Dy₂O₃ as sintering additives respectively. The effects of three kinds of sintering additives on the phase composition, microstructure and thermal conductivity of AlN ceramics were investigated. The results showed that those sintering additives not only facilitated the densification via the liquid phase sintering mechanism, but also improved thermal conductivity by decreasing oxygen impurity. Sm₂O₃ could effectively improve thermal conductivity of AlN ceramics compared with Y₂O₃ and Dy₂O₃. Observation by scanning electron microscopy showed that AlN ceramics prepared by spark plasma sintering method manifested quite homogeneous microstructures, but AlN grain sizes and shapes and location of secondary phases varied with the sintering additives. The thermal conductivity of AlN ceramics was mainly affected by the additives through their effects on the growth of AlN grain and the location of secondary phases.     

The study of AlN ceramics sintered at superhigh pressure with belt-type press technology

The effects of Y₂O₃ content, sintering time, sintering temperature, sintering pressure on thermal conductivity of AlN ceramics had been studied. X-ray diffraction (XRD), scanning electron microscope (SEM), laser conductometer and laser granularity dimension analysis measurer were respectively used to measure the phases, microstructure, thermal conductivity and particle size distribution of the samples. These studies reveal that the Y₂O₃ is an effective sintering addtive, and the best conditions of sintering are that the pressure is 5.15× 10⁹ Pa, the temperature is 1700∘C and the sintering time is 115 min. Under these conditions, the sintered body has reasonable structure and its thermal conductivity is 200 w/(m⋅k).     

High dielectric AlN/Al2O3 composite powder using a plasma spray approach for microelectronics application

Increasing demand for higher performance dielectric material for multi-layer ceramics packaging has led to the use of the AlN system due to its very high thermal conductivity and coefficient of expansion compatibility with silicon. This paper reports on a novel process method used to produce an AlN/Al₂O₃ composite powder system which can be subsequently tape cast as a dielectric substrate. The mixture of both Al₂O₃ and AlN was first mechanically alloyed and then spray-dried to obtain a suitable agglomerated powder that was subsequently plasma-sprayed, resulting in a fine micrometer level integrated composite powder. The two main criteria used to ascertain the optimal process parameters during plasma spraying were a high gamma/alpha Al₂O₃ phase ratio, which ensured that all the Al₂O₃ phase had melted during plasma spraying, and a minimal reduction in the AlN/Al₂O₃ ratio to ensure minimal change in the AlN during processing. For the plasma-sprayed composite powders, fully sintered ceramic tapes were produced attaining>99.0% of the theoretical density after sintering at 1650°C for 6 h, which yielded a thermal conductivity value of 32.0 W m^–1 K^–1.     

Migration of liquid phase in low temperature sintering of AlN

In the process of low-temperature sintering of AlN ceramics, the reaction of the sintering aids YF₃ and CaF₂ with superficial Al₂O₃, inherently contained in AlN lattice, results in formation of liquid phase. Nevertheless, the uniformly dispersed liquid phase is prone to migrate from the bulk to the surface of the samples, opposing densification. The analysis of the experimental results indicates that fresh liquid phase can continuously arrive from the bulk to the surface due to chemical reactions and crystallization which occur at the surface as well as wettalibilty and capillarity phenomena. The surface is depleted of liquid phase since the latter is consumed due crystallization and carbothermal reduction reactions with the elements of the atmosphere of the furnace N₂ and C, resulting in formation of a dense layer of crystals of Al₂Y₄O₉, CaYAl₃O₇ and Y₂O₃, grown perpendicularly to the surface. The chemical and structural features of this newly formed crystalline surface layer generate a significant difference of the wetting regimes and the capillary forces between the surface and the bulk, favouring pumping of the liquid from the bulk to the surface.     

Pressureless sintered SiAlON with low amounts of sintering aid

-SiAlONs of compositions Si_2.6Al_0.393Y_0.007O_0.4N_3.6 and Si_2.6Al_0.384Y_0.014O_0.4N_3.6 were pressureless sintered from mixtures of Y₂O₃ and separately milled -Si₃N₄, AlN, and SiO₂. On sintering, the carbon content of these SiAlONs was reduced to negligible levels and their oxygen content was also proportionately reduced, presumably due to reaction of carbon with SiO₂. These SiAlONs had densities in excess of 98% of theoretical, four-point bend strengths of 460 and 155 MN m^–2 at r.t. and 1400° C, respectively, and 1400° C oxidation rates lower than those reported in the literature for hot-pressed Si₃N₄ and for a similar but stronger SiAlON with 2.5 wt % Y₂O₃. These results indicate that increasing the Y₂O₃ content of SiAlONs increases their strength but decreases their oxidation resistance.     

Mechanical properties and microstructure of reaction sintered β′-sialon ceramics prepared by a slip casting method

Reaction sintered β′-sialon ceramics Si_6-zAl_zO_zN_8-z, were prepared by slip casting from α-Si₃N₄, Al₂O₃, and AlN starting powders. The mechanical properties and microstructures of sintered bodies were investigated as a function of composition (varying the z value). The maximum value of the flexural strength, ∼ 600 MPa, and fracture toughness, ∼ 4.1 MPa m^1/2 were observed in the z range of 0.5–1. In the z value range of 2–4, the mechanical properties decreased drastically. This phenomenon is attributed to the variation of fracture energy, which is greatly affected by the sintered crystallite size. This revised version was published online in November 2006 with corrections to the Cover Date.     

Joining of AlN and graphite disks using interlayer tapes by spark plasma sintering

AlN and graphite disks were successfully joined using a polymer plasticized ceramic tape as the interlayer by spark plasma sintering (SPS). The tape contains either composite powders of AlN and graphite or AlN powders without graphite. Both tapes contained 5 mass% Y₂O₃ as the sintering aid of AlN. The joining was carried out at 1700–1900 °C and 30 MPa for 5 min. No other reaction phase except for Al₂Y₄O₉ was identified in the joints. The maximum tensile strength of the joints was obtained when the AlN–graphite composite interlayer tape was used. The joining mechanism is attributed not to the chemical bonding, but to the physical bonding of the Al₂Y₄O₉ phase, which is solidified from the molten Al–Y–O squeezing into the porous graphite under pressure during SPS.     

Effect of grain width and aspect ratio on mechanical properties of Si<Subscript>3</Subscript>N<Subscript>4</Subscript> ceramics

Sintering additives Y₂O₃ and Al₂O₃ with different ratios ((Y₂O₃/Al₂O₃) from 1 to 4) were used to sinter Si₃N₄ to high density and to induce microstructural changes suitable for raising mechanical properties of the resultant ceramics. The sintered Si₃N₄ ceramics have bi-modal microstructures with elongated β-Si₃N₄ grains uniformly distributed in a matrix of equiaxed or slightly elongated grains. Pores were found within the grain boundary phase at the junction regions of Si₃N₄ grains. The highest average aspect ratio (length/width of the grains) of ∼4.92 was found for Y₂O₃/Al₂O₃ ratio of 2.33 with fracture toughness and strength values of ∼7 MPam^1/2 and 800 MPa, respectively. The effect of microstructure, specifically grain morphology, on mechanical properties of sintered Si₃N₄ were investigated and found that the aspect ratio of the elongated grains is the most important microstructural feature which controls mechanical properties of these ceramics.     

Reaction sintering fabrication of (AlN, TiN)-Al2O3 composite

The (AlN, TiN)-Al₂O₃ composites were fabricated by reaction sintering powder mixtures containing 10-30 wt.% (Al, Ti)-Al₂O₃ at 1420-1520°C in nitrogen. It was found that the densification and mechanical properties of the sintered composites depended strongly on the Al, Ti contents of the starting powder and hot pressing parameters. Reaction sintering 20 wt.% (Al, Ti)-Al₂O₃ powder in nitrogen in 1520°C for 30 min yields (AlN, TiN)-Al₂O₃ composites with the best mechanical properties, with a hardness HRA of 94.1, bending strength of 687 MPa, and fracture toughness of 6.5 MPa m^1/2. Microstructure analysis indicated that TiN is present as well dispersed particulates within a matrix of Al₂O₃. The AlN identified by XRD was not directly observed, but probably resides at the Al₂O₃ grain boundary. The fracture mode of these composites was observed to be transgranular.     

Structural and Electrical Properties of Bismuth Ferrite Ceramics Sintered in Different Atmospheres

Bismuth ferrite (BiFeO₃) ceramics were synthesized by the solid-state reaction method followed by rapid liquid phase sintering. The effect of sintering atmosphere (N₂, air and O₂) on the structure and electrical properties of BiFeO₃ multiferroic ceramics were investigated. XRD analysis revealed that N₂ sintering was effective in reducing impurity phases and improving the crystallization behavior. XPS analysis showed that fewer Fe²⁺ ions but more oxygen vacancies were involved in the N₂ sintered ceramics. The SEM investigations suggested that the grain size of the BiFeO₃ ceramics sintered in nitrogen are larger than those sintered in air and O₂. Electrical measurements revealed that the ceramics sintered in N₂ showed lower leakage current, superior dielectric and ferroelectric properties.     

Synthesis and properties of in situ Si3N4-reinforced BaO·Al2O3·2SiO2 ceramic matrix composites

Silicon nitride (94.5% α, 5.5% β), BaCO₃, Al₂O₃, and SiO₂ powders were mixed and pressureless sintered to produce a ceramic matrix composite consisting of 30 vol% barium aluminosilicate (BaO·Al₂O₃·2SiO₂ or BAS) matrix reinforced with in situ grown whiskers of β-Si3N4. In situ X-ray studies of the reactions indicated that BaCO₃ decomposes first to yield BaO which reacts with SiO₂ to yield a series of barium silicates which then react with Al₂O₃ between 950 and 1300°C to yield hexacelsian BAS. The sintering times were varied in order to develop a material system that combines the favourable properties of BAS with the high strength of Si₃N₄. In situ high-temperature X-ray studies after composite processing did not reveal any changes in the BAS or Si3N4 up to temperatures of 1300°C. Dilatometry studies of the sintered composite indicated a low-temperature transformation between 230 and 260°C with the temperature of transformation and volume change associated with the hexagonal to orthorhombic transformation decreasing with an increase of sintering time. Room- and high-temperature (1400°C) strengths were evaluated using four-point bend flexural tests. Composites exhibited near theoretical densities and an increase in flexural strength that was primarily dependent on the higher α- to β-Si₃N₄ transformation. This revised version was published online in November 2006 with corrections to the Cover Date.     

Direct bonding of Cu to oxidized silicon nitride by wetting of molten Cu and Cu(O)

When pressureless sintered silicon nitride with the main additives Y₂O₃ and Al₂O₃, having a thermal conductivity K = 20 W/m K, was oxidized at 1240–1360 °C in still air, the resulting surface oxide layer easily bonded to a copper plate in the temperature region between 1065 and 1083 °C, and in the oxygen concentration range of 0.008–0.39 wt%, as shown in a Cu–O phase diagram. The oxide on the silicon nitride was characterized as Y₂O₃·2SiO₂ and mixed silicate glass with additives and impurities that diffused through the grain boundary. The bonding strength of Cu/Si₃N₄ depends on the amount or layer thickness of silicate glass and reaches as high as 100 MPa by shear at room temperature. Detailed analysis of the oxidation layer and the peeled-off surfaces of directly bonded Si₃N₄/Cu reveal that the main mechanism of bonding is wetting to glassy silicate phase by mixtures of molten Cu and α-solid solution Cu(O), which solidify to α + Cu₂O below 1065 °C by a eutectic reaction. The direct reactive wetting of molten Cu, supplied from the grain boundary of a Cu plate, on the glassy phase enables very tight chemical bonding via oxygen atoms.     

Reactive and dense sintering of reinforced-toughened B4C matrix composites

Reactive hot-press (1800-1880 °C, 30 MPa, vacuum) is used to fabricate relatively dense B₄C matrix light composites with the sintering additive of (Al₂O₃ +Y₂O₃). Phase composition, microstructure and mechanical properties are determined by methods of XRD, SEM and SENB, etc. These results show that reactions among original powders B₄C, Si₃N₄ and TiC occur during sintering and new phases as SiC, TiB₂ and BN are produced. The sandwich SiC and claviform TiB₂ play an important role in improving the properties. The composites are ultimately and compactly sintered owing to higher temperature, fine grains and liquid phase sintering, with the highest relative density of 95.6%. The composite sintered at 1880 °C possesses the best general properties with bending strength of 540 MPa and fracture toughness of 5.6 MPa m^1/2, 29 and 80% higher than that of monolithic B₄C, respectively. The fracture mode is the combination of transgranular fracture and intergranular fracture. The toughening mechanism is certified to consist of crack deflection, crack bridging and pulling-out effects of the grains.     

The effects of pre-oxidation on the sintering and mechanical property of powder injection moulded SiC material

Usually, injection moulded SiC green parts are debound in inert atmosphere or vacuum, which induces the residual carbon and increases forming cycle and production cost. In this paper, injection moulded SiC with Al₂O₃ and Y₂O₃ as sintering assistant was thermal debound in air and Ar, respectively. The paper investigates the effects of pre-oxidation during debinding stage on the sintering and mechanical property of SiC material. During sintering, the oxide SiO₂ is in favour of the shrinkage of debound samples at lower temperature. After sintering, the linear shrinkage of sintered samples with pre-oxidation is bigger than the sample without pre-oxidation. Test results by TEM and XRD indicate that SiO₂ disappear from the inside of the sintered samples. The loss of SiO₂ decreases the content of Al₂O₃, which affects the formation of YAG (Y₃Al₅O₁₂). Sintered Sic samples contain α-SiC phase and intergranular phase. There is no hetero-phase between the boundaries of α-SiC phase and intergranular phase. The bending and compression strength values of sintered samples with pre-oxidation reach to 537 MPa and 2.89 GPa, respectively. These values approach the strength of sintered samples without pre-oxidation (594 MPa and 3.0 GPa).     

Thermal expansion coefficient of Y-α′-sialon

A sialon composite composed of Y-α′-sialon and β′-sialon has been fabricated by hot pressing mixtures of Si₃N₄, Y₂O₃ and AlN powders. Thermal expansion coefficients of the Y-α′-sialon and β′-sialon were determined by the high-temperature X-ray diffraction technique. The thermal expansion coefficient of Y-α′-sialon depended on the composition, being minimum at x=0.3 in the formula Y_x(Si_12-4.5x, Al_4.5x)(O_1.5x, N_16-1.5x). The coefficient of β′-sialon increased with increasing lattice constant, that is, the z value in the formula Si_6-zAl_zO_zN_8-z. The thermal expansion coefficient of sialon composites determined by a differential dilatometer increased with increasing amount of Y-α′-sialon. This revised version was published online in November 2006 with corrections to the Cover Date.